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Flame ionization detectors (FIDs) use a hydrogen flame to produce a specific type of carbon ion. Just as with the PID, ions allow flow of electrical current. They detect the low-end concentrations of organic vapors and gases from 0.1 to 1,000 ppm. Limitations include low oxygen levels, cold temperatures and lower molecular weight hydrocarbon derivatives.
Gas chromatography (GC) sensor technology operates through differential separation (through the column) by the molecular weight of the contaminant. The mixture then passes into a flame ionization sensor. As different quantities of different molecularly weighted substances pass through the ionization chamber, their combustion is charted. Gas chromatography is used for the differentiation of mixtures, as well as the confirmation and identification of substances. It is also used for the identification of known possible contaminants. Limitations: operating temperature ranges average between approximately 45° F (7.2° C) and 105° F (41° C); unless one is using portable field units, one cannot provide readings in the field; gas chromatography is only intended to monitor organic compounds; and it requires a “footprint” (graph or GC run) on file to identify substances.
Ion mobility spectrometry (IMS) systems measure how fast a given ion moves in a uniform electric field through a given atmosphere. Sample molecules are ionized in numerous ways, including corona discharge (electrical arc) and radioactive sources. Samples of the ions are let into a drift chamber; once in the drift tube, ions are subjected to an electrical field. This electric field then drives the ions through the drift tube where they interact with the neutral drift molecules contained within the system. Chemical identification is based on the ion mobility (ion mass, size and shape) where they arrive at the detector for measurement. Ions are recorded at the detector in order from the fastest to the slowest, generating a response signal characteristic for the chemical composition of the measured sample. Ion mobility spectrometry is fast, highly sensitive, easy to use and compact. IMS can be used for the field detection of numerous materials, including explosives, drugs and chemical weapons.
Surface acoustic wave (SAW) sensors are made by using a substrate material, quartz or lithium tantalate and bonded to a metal. This is then coated with a chemically absorptive material. Based on specific combinations of these materials and electrical charge, these sensors become highly sensitive to mechanical fluctuations (vibrations/acoustic waves) created by airborne substances attached to the surface. A surface acoustic wave makes for a very dependable and selective detector when multiple surface acoustic wave sensors with various absorptive coating are put in combination with pattern recognition software.
Spectroscopy in the field
Infrared spectroscopy detects the vibration characteristics of chemical functional groups in a sample. When an infrared light interacts with the sample, chemical bonds will stretch, contract and bend. Chemical functional groups tend to adsorb infrared radiation in a specific wave number range regardless of the structure of the rest of the molecule. The wave number positions where functional groups adsorb are consistent and not affected by temperature, pressure, sampling or change in the other parts of the molecular structure. Limitations: knowing what the substance is likely to be; must have footprint for known substances as a cross-reference; requires substantial maintenance and calibration; and interference gases can produce inaccurate readings. Infrared spectroscopy detects covalently bonded materials. It can be used to monitor one or more known substances and in the identification of compounds by comparison to known spectral data.
Raman spectroscopy technology is an excellent complement to infrared spectroscopy. Both Raman and infrared work well for identification of unknown substances. A higher percentage of positive identifications of unknown substances can be attained when both technologies are used. This leads to higher user confidence. Light (monochromatic/laser/single frequency) impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, which generates scattering. Single frequency light is scattered at different frequencies and collected with an array detector. For the Raman to be sensitive, the sample must have a polarizable covalent chemical bond.